In many industries it is required to set up an array of equipment at a temporary work site, and then disassemble the installation, move it to another work site, and re-assemble it. For example asphalt and concrete plants are often set up in locations where large quantities of material are required for mad or building construction. Similarly well drilling rigs, such as those drilling for oil and gas, are set up at a drilling location, then taken down and moved to a different drilling location.
These portable plants and rigs typically include various tanks, bins, silos, and like containers for water, asphalt, sand, cement, and other materials such as might be required at any particular work site. These are transported to the work site, commonly in a lowered horizontal transport orientation, and then raised to a vertical working orientation at the work site. These containers are typically transported to the work site by flat bed trucks or trailers in a lowered horizontal transport orientation. The trailer is maneuvered into position, and then the container is raised from the horizontal transport position to a vertical working position resting on the ground, and the trailer is moved away from the site.
This vertical orientation reduces the ground area, or footprint, occupied by the installation. Also bins containing dry granular material especially are often elongated vertically when in a working position to maximize the quantity of material stored above a hoppered bottom such that the granular material will flow out by gravity. Containers holding granular material typically have a hoppered bottom with walls sloping toward an outlet port at the bottom of one side of the container so that all the material inside will flow out through the outlet port, leaving the container empty.
The outlet port is in a fixed location on one side of the bottom of the container, and so during setup at a work site the container must be oriented properly so the outlet port is in a location where the material can be received from the outlet port and used in whatever operation is occurring. Commonly the container includes a chute under the outlet port to direct granular material flowing from the port onto a conveyor belt or the like.
As granular material flows out of the outlet port, the material above the port inside the container flows out first and the level of granular material inside the container on the port side thereof above the fixed outlet port falls until the angle of repose of the particular material is reached, and material from the opposite side of the container begins to flow. Thus the loaded container is unbalanced, with more weight on the side opposite the port, making the container less stable, as the wind force necessary to be exerted on the port side of the container to tip the container over is reduced.
Such containers are also known that have a center outlet port in the bottom, however a conveyor or the like is then required to receive the material and carry it to a location outside the container. Alternatively the container can be mounted on legs and a chute can receive the material and direct it to a location beside the container, however this makes containers of the same capacity considerably higher and less stable.
Stability is also reduced when the container is empty. When filled, the weight of the contents keeps the container in place but when empty, winds or accidental contact with other equipment being moved at the work site can more easily cause the container to tip over. The stability of such containers is commonly calculated as an empty wind load rating based on the wind velocity necessary to tip the container over when empty.